The present invention relates to an integrated vertical resistor structure with reduced dimensions, for high voltage, and to a manufacturing process thereof.
As is known, high-voltage resistors are widely used in integrated power devices. For example, in devices which are formed using VIPower technology, a single semiconductor material chip houses power devices subject to high-value operative voltages (for example up to 2 KV), and control devices which normally operate with voltages lower than a few tens of volts.
In particular, in some applications, it is necessary to have voltages for biasing the control devices, which are obtained from the biasing voltage of the substrate. To provide a partition supplying the required voltage values, a resistor can be connected between the control devices, which operate at low voltage, and the substrate, which, like the power devices, can be biased to voltages of a few Kilovolts. The resistor must have a high resistance value, for example comprised between 100 Kxcexa9 and a few Mxcexa9.
According to a known, commonly used solution, to form the high-voltage resistors, in a semiconductor material substrate a doped region with high resistivity is formed, which has conductivity opposite that of the substrate, and which also, in plan view, has the shape of a coil, with a plurality of branches which are parallel to one another, and adjacent in pairs.
However, this solution is disadvantageous owing to the required area. In fact, to prevent malfunctioning, adjacent branches of the coil resistor must be appropriately spaced, depending on the doping of the substrate and on the voltage applied to the ends of the resistor. This is due to the fact that, when an inverse voltage is applied to a junction region between conductive regions with opposite conductivity (as is the case between high-voltage resistors and the substrate), depletion areas are formed within the conductive regions and have an amplitude which depends on doping, as well as on the value of the applied inverse voltage. In particular, the dimensions of the depletion areas decrease as the concentration of doping agent increases, whereas it increases when the inverse voltage applied is increased.
Since, to be able to withstand high voltages, the substrate must be highly resistive, and therefore is formed by semiconductor material with a low concentration of doping agent, it is clear that the depletion areas in the substrate are very extensive. In particular, near the terminal of the resistor which is connected to the control devices, where the voltage drop between the resistor and the substrate is greater, it may happen that the depletion areas that are formed in the substrate at two adjacent branches of the coil resistor come into contact with one another, thus giving rise to pinch-off. This phenomenon causes significant deterioration of the resistance of the resistor, and therefore can adversely affect the correct operation of the entire circuit.
To prevent pinch-off from occurring, a minimum distance should be present between two adjacent branches of the resistor, the distance being no less than the sum of the amplitudes of the depletion areas associated with each of the two branches.
The large bulk of the coil resistor is also due to the fact that the high voltages which are applied to the resistor require edge structures for protection against premature breakdown in the regions of the resistor which are most subject to high voltages. For this purpose, metal field plates are formed, i.e., annular regions with a high resistivity, which surround the coil resistor.
A further factor which increases the bulk of a resistor of the above-described type, is its interaction with the edge structures of the devices to which it is connected, such that the resistor must be formed near the terminal region of the device supplying the high voltage.
To reduce the depletion area between the various branches of the coil resistor, a solution described in European patent application 98830638.7, filed on Oct. 23, 1998 by the same applicant, is based on forming the coil resistor from a highly resistive semiconductor material layer, which has conductivity opposite that of the substrate, and of forming one or more isolation trenches between each pair of adjacent parallel branches of the coil resistor. These isolation trenches, for example of silicon dioxide, extend in the substrate to a greater depth than the semiconductor material layer of the coil resistor, by an extent sufficient to prevent pinch-off.
However, according to this solution, the coil resistor is located close to the terminal region of the device supplying the high voltage, and consequently the obtainable reduction in bulk is relatively small, and the disadvantageous interaction between the resistor and the edge structures of the device integrating the resistor is still present.
According to the present invention, an integrated, semiconductor material device is provided, having a high-voltage resistor including a high-voltage region and a low-voltage region positioned above said high-voltage region, said high-voltage and low-voltage regions having a first conductivity type.
An isolation region, at least partially buried, and extending between said high-voltage region and said low-voltage region. The isolation region is interrupted at, and laterally delimiting, a vertical resistive region, which connects said high-voltage region to said low-voltage region.
According to the invention, a method for manufacturing an integrated device of semiconductor material comprising a high-voltage resistor is also provided, the method comprising the steps of:
forming a high-voltage region having a first conductivity type; and
forming a low-voltage region having said first conductivity type, above said high-voltage region;
characterised by the step of forming an isolation region, at least partially buried, and arranged between said high-voltage region and said low-voltage region; said isolation region being interrupted at a vertical resistive region, which connects said high-voltage region to said low-voltage region.